1. Field of the Invention
The present invention generally relates to a surface processing technique of a silicon wafer substrate. Particularly, this invention relates to a technique for directionally removing or dry etching of an oxide film, using a plasma of NH.sub.3 and NF.sub.3 or CF.sub.4 and O.sub.2 mixed with H.sub.2 and N.sub.2.
2. Description of the Prior Art
The exposure of a surface of a silicon (Si) substrate to ambient air causes a layer of native silicon dioxide (SiO.sub.2) to form on its surface. The formation of the native SiO.sub.2 layer presents certain manufacturing difficulties since many semiconductor processes, such as low-temperature epitaxy, polysilicon deposition, and silicidation, require the silicon wafer surface to be free of all native SiO.sub.2. The prior art establishes that many different processes for etching or cleaning of the native SiO.sub.2 from Si surfaces have been developed. One of the most commonly used techniques for etching SiO.sub.2 is to expose the substrate to hydrofluoric acid (HF), either wet or dry. However, the use of liquid or vapor HF has major drawbacks for the semiconductor manufacturer because it is both highly toxic and highly corrosive. The high toxicity of HF requires the semiconductor manufacturer to implement safety procedures and devices which ensure that people and the environment are not injured by the HF gas. In addition, since HF is highly corrosive, it must be stored in containers and delivered to a cleaning or etch chamber in tubing which is resistant to corrosion. This type of equipment is generally more expensive than containers and tubing that do not possess anti-corrosive properties.
It is also known in the prior an to selectively etch silicon dioxide using mixtures of fluorocarbon gases. However, this technique is also problematic due to the reaction inhibiting polymer which forms when the silicon or silicon nitride substrate is exposed to the fluorocarbon gases. In this technique, when oxide film is present, the oxygen in the oxide film prevents the formation of a reaction inhibiting polymer. However, once the oxide is etched through, the reaction inhibiting polymer can form and is deposited on the silicon or silicon nitride, inhibiting the etching of these films. The reaction inhibiting polymer film is difficult to remove from the wafer. The polymer film can also passivate the chamber wall and electrode surfaces and is a source of particles. Finally, the etch selectivity, in this process, is under 20:1 for SiO.sub.2 relative to polysilicon or silicon nitride.
The use of wet etching techniques, as described above, to remove a thermal or native oxide film also present problems due to the variation in the wet etch rates of different oxide films. There are many types of oxide films currently in use, with two common ones being thermal oxide and deposited tetraethylorthosilicate TEOS) oxide. The etch rate of these films can vary by a factor of three, with the thermal oxides being slower. Since each wet etch performed reduces the film thickness of any exposed deposited oxide in proportion to the etch rate milo, it is usually necessary to adjust the initial thickness of the deposited oxide. Therefore, it would be desirable for the deposited oxides to etch at the same or a lower rate than the thermal oxides.
U.S. Pat. No. 5,030,319 to Nishino et al. and in the article to Nishino et al., 1989 Dry Process Symposium IV-2 90-92 (1989), discloses a method of selectively etching native SiO.sub.2 from a silicon substrate using fluorine atoms and nitrogen hydrides produced by a NH.sub.3 and NF.sub.3 microwave discharge. This technique, as shown in FIG. 1A, discloses the use of a source 12 remotely positioned from etch chamber 16 to excite a gaseous NH.sub.3 and NF.sub.3 mixture into a plasma prior to its transport to the etch chamber 16. The substrate 18 is located on the floor 20 of etch chamber 16. The remote excitement of the gases causes the excited gases to randomly diffuse over the substrate which is to be etched. There is no directional control over the etching process. FIG. 2B provides an example of etching using this method. Since, the excited particles etch in all directions an isotropic product is created. As shown, the process is effective in etching to the surface of the substrate 52, but results in the undercutting of the wall 58 in the area beneath resist pattern 54 which should not to be etched. It is desirable for the etching product to have an anisotropic profile.